Calcium plays an indispensable role in cell permeability, the formation of bones and teeth, blood coagulation, transmission of nerve impulse, and normal muscle contraction. The concentration of calcium ions in the blood is, along with calcitriol and calcitonin, regulated mainly by parathyroid hormone (PTH). Extracellular calcium levels are directly affected by PTH through calcium uptake in kidney tubule cells and calcium transport to or from bone. Although calcium intake and excretion may vary, PTH serves through a feedback mechanism to maintain a steady concentration of calcium in cells and surrounding fluids. When serum calcium lowers, the parathyroid glands secrete PTH, affecting the release of stored calcium. When serum calcium increases, stored calcium release is retarded through lowered secretions of PTH.
The complete or whole form of human PTH, (hPTH), is a unique 84 amino acid peptide (SEQ ID NO:5), as is shown in FIG. 1. Researchers have found that this peptide has an anabolic effect on bone that involves a domain for protein kinase C activation (amino acid residues 28 to 34) as well as a domain for adenylate cyclase activation (amino acid residues 1 to 7). However, various catabolic forms of clipped or fragmented PTH peptides also are found in circulation, most likely formed by intraglandular or peripheral metabolism, or example, hPTH can be cleaved between amino acids 34 and 35 to produce a (1-34) PTH N-terminal fragment and a (35-84) PTH C-terminal fragment. Likewise, clipping can occur between either amino acids 36 and 37 or 37 and 38. Recently, a large PTH fragment referred to as “non-(1-84) PTH” has been disclosed which is clipped closer to the N-terminal end of PTH. (See R. LePage et alia, “A non-(1-84) circulating parathyroid hormone (PTH) fragment interferes significantly with intact PTH commercial assay measurements in uremic samples ” Clin Chem (1998); 44: 805-810.).
The cleaved fragments of PTH vary in both biological activity and metabolic clearance rate from the circulation. For example, the N-terminal human PTH1-34 (hPTH1-34) fragment has PTH agonist properties, but is rapidly removed from circulation. A daily subcutaneous administration of hPTH to patients with idiopathic osteoporosis has been shown to substantially increase their iliac trebecular bone volume. (See R. Podbesek et alia, “Effects of two treatment regimes with synthetic human parathyroid hormone fragment on bone formation and the issue balance of trabecular bone in greyhounds”. Endocrinology (1983); 112: 1000-1006.)
Osteoporosis is the most common form of metabolic bone disease and may be considered the symptomatic, fracture stage of bone loss (osteopenia). Although osteoporosis may occur secondary to a number of underlying diseases, 90% of all cases appear to be idiopathic. Postmenopausal women are particularly at risk for idiopathic osteoporosis (postmenopausal or Type I osteoporosis). Another high risk group for idiopathic osteoporosis is the elderly of either sex (senile or Type II osteoporosis). Osteoporosis has also been related to corticosteroid use, immobilization or extended bed rest, alcoholism, diabetes, gonadotoxic chemotherapy, hyperprolactinemia, anorexia nervosa, primary and secondary amenorrhea, and oophorectomy.
In the various forms of osteoporosis, mechanical failure bone fractures frequently occur which are the result of bone loss. Postmenopausal osteoporosis is characterized by fractures of the wrist and spine, while femoral neck fractures seem to be the dominant feature of senile osteoporosis.
Bone loss in osteoporotics is believed to involve an imbalance in the process by which the skeleton renews itself This process has been termed bone remodeling. It occurs in a series of discrete pockets of activity. These pockets appear spontaneously within the bone matrix on a given bone surface as a site of bone resorption. Osteoclasts (bone dissolving or resorbing cells) are responsible for the resorption of a portion of bone of generally constant dimension. Resorption is followed by the appearance of osteoblasts (bone forming cells) that refill the cavity left by the osteoclasts with new bone.
In a healthy adult subject, the rate at which osteoclasts and osteoblasts are formed is such that bone formation and bone resorption are in balance constituting an optimal bone turnover rate. However, in osteoporotics an imbalance in the bone remodeling process develops which results in bone being lost at a rate faster than it is being made. Although this imbalance occurs to some extent in most individuals as they age, it is much more severe and occurs at a younger age in postmenopausal osteoporotics or following oophorectomy.
There have been many attempts to treat osteoporosis with the goal of either slowing further bone loss or, more desirably, producing a net gain in bone mass. Certain agents, such as estrogen and the bisphosphonates, appear to slow further bone loss in osteoporotics. Agents which slow bone loss, because of the different durations of bone resorption and formation, may appear to increase bone mass (on the order of 3% to 7%). However, this apparent increase is limited in time, not progressive, and is due to a decrease in “remodeling space.” In addition, because of the close coupling between resorption and formation, impeding bone resorption also ultimately impedes bone formation.
Another class of agents investigated to combat the onset of osteoporosis encompasses PTH and PTH analogues. (See U.S. Pat. No. 6,051,686 to Kretenansky et alia, for example.) The theory behind the use of such compounds is to use the body's natural protein receptor binding process to counter a greater removal of calcium from bone than resorption of calcium. Unfortunately, such proposed treatments have had adverse effects, including hypercalcemia (elevated serum calcium) and the formation of osteosarcomas.